1. Field of the Invention
The present invention relates to a method and apparatus for controlling the steering of the rear wheels of a motor vehicle.
2. Description of the Related Art
In order to improve the maneuverability and stability of a motor vehicle, it is known in the art to steer all of the front and rear wheels in accordance with an operation of the steering wheel. In a typical example of four-wheel steering, the rear wheels are steered to the same phase side as the front wheels when the vehicle is turning in medium- or high-speed region. According to the four-wheel steering, the swerving motion of the vehicle (the turning motion of the forepart) is converted to lateral or side-to-side motion and thus the influence of the moment of inertia lessens, whereby the response delay and tail swing (yawing) during turning of the vehicle are reduced, improving the vehicle traveling stability.
However, even if the four-wheel steering technique is employed, the vehicle traveling stability can be lowered when the load weight of the vehicle is increased. This is because the ground load on the rear-wheel side is great as compared with the front-wheel side.
To avoid such a lowering of the traveling stability attributable to increase in the load weight, it is known to increase the steering ratio (the ratio of the rear-wheel steering angle to the front-wheel steering angle) with increase in the load weight, as disclosed in Examined Japanese Patent Publication (KOKOKU) No. 5-25710.
In Unexamined Japanese Patent Publication (KOKAI) No. 2-81771 is disclosed an apparatus for obtaining a target rear-wheel steering angle .theta.r* according to the formula .theta.r*=(a..theta.f(t)+b..theta.f(t-.DELTA.t)/(a+b). In this apparatus, as the load weight increases, the value of the coefficient "a" is increased and at the same time the value of the coefficient "b" is decreased. Consequently, when the load weight is small, the rate of the steering quantity component b..theta.f(t-.DELTA.t)/(a+b), which is delayed from the front-wheel steering by time .DELTA.t, with respect to the target rear-wheel steering angle .theta.r* becomes greater, thereby increasing the steering response of the vehicle. When the load weight is large, on the other hand, the rate of the steering quantity component a..theta.f(t)/(a+b), which has no delay with respect to the front-wheel steering, becomes greater. This thereby restrains collapse of the goods on the vehicle and prevents passengers from receiving an uncomfortable feeling.
However, if the rear-wheel steering angle is increased with increase in the load weight, as in Japanese Patent Publication(Kokoku) No. 5-25710, the side-to-side motion component of the vehicle increases while the swerving (turning) motion component decreases. This potentially spoils the steering feeling (making the driver feel it difficult to turn). Further, energy consumption increases.
In Japanese Patent Publication(Kokoku) No. 2-81771, the substantial response delay time for the rear-wheel steering is shortened as the load weight increases. This thereby controls the steering response (yaw response) of the vehicle in accordance with the load weight. When the load weight is small, however, the steering response becomes unnecessarily high even if the driver slowly operates the steering wheel. On the other hand, when the load weight is large, high steering response is not available even if the driver quickly operates the steering wheel. Thus, in these known systems, the steering response changes only uniformly in accordance with the load weight, and the driver's steering operation (steering frequency) is not at all taken into consideration.